The present invention relates generally to methods and compositions for performing dynamic nuclear polarization (DNP), and for further performing Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) using the compounds so produced. NMR has become an irreplaceable tool in the studies of macromolecular structures, dynamics, and interactions, such as in the studies of proteins, DNA, their complexes, and also in imaging tissue structures. However, a major bottle neck of NMR for high throughput applications is its low sensitivity. The intrinsic reason for low sensitivity arises from the low equilibrium polarization of Boltzmann populations at different energy levels of nuclear spins in an external magnetic field. The equilibrium polarization is proportional to the strength of the external magnetic field and inversely proportional to the absolute temperature of the sample. Thus, to increase the polarization, one can either increase the strength of the external magnetic field, decrease the sample temperature, or do both.
To date, the highest field of commercially available magnet is ˜23.5 Tesla (corresponding to proton magnetic resonance frequency ˜1 GHz). To increase the magnetic field strength of the highest commercial magnet even by two-fold (to 47 Tesla) cannot currently be technologically and/or financially foreseen. Another way to handle the problem of low sensitivity is to develop more sensitive instruments to detect NMR signals. For example, CryoProbe™ devices developed in recent years have been able to increase NMR sensitivity up to four-fold. CryoProbe™ device technology deals with suppressing the instrumental thermal noises by reducing the temperature of the NMR coil and preamplifier using cold helium gas at ˜20K. In order to tremendously increase the NMR signal intensities, other technologies and methods have yet to be developed.
A promising approach to increasing NMR sensitivity is to dynamically polarize the nuclear spins of the samples. If a sample contains either exogenous or endogenous paramagnetic centers, such as paramagnetic metal ions (including transition, actinide, and lanthanide metal ions), nitroxide radicals, and trityl radicals, for example, irradiation of the paramagnetic centers in a magnetic field can lead to dynamic polarization of the nuclear spins. In principle, this technique can enhance the proton signal by 660-fold and that of 13C by 2600-fold due to the gyromagnetic ratios of electron to nuclei (γe/γn) of proton and 13C, respectively. Use of dinitroxide-type biradical compounds for dynamic nuclear polarization (DNP) previously has been described. A factor of signal enhancement of 175-fold was achieved. The DNP experiments are carried out at temperatures ≦100° K in order to suppress the electronic and nuclear spin-lattice relaxation rates to favor the polarization transfer. At such a low temperature, a solvent is required for homogenous distribution of the paramagnetic centers in the frozen solution for effective polarization distribution throughout the sample. The most commonly used solution is a 40:60 v/v mixture of water:glycerol, which forms a glassy matrix at low temperature regardless of the cooling rate. Biological samples are also expected to be cryo-protected using this solvent.
Despite the achievement of the DNP experiments, one drawback is the use of the mixture of water and glycerol as the solvent. This solvent requirement may limit many biological applications because in almost all of the studies of biomolecules, water represents the sole environment to mimic the behaviors and structures of biomolecules as in living cells. This invention addresses the issue of using water as the primary solvent for DNP at low temperatures through the use of electron spin labeled ice binding compounds (IBCs).